MPs will be lobbied after a rally opposing the government’s plans to part-privatise Royal Mail later today.

Business secretary Lord Mandelson is expected to launch a bill pushing through the sell-off of a sizeable stake in the postal services provider on Thursday.

The move comes amid fierce opposition from over 120 Labour MPs, who have signed an early day motion tabled by Geraldine Smith which states that part-privatisation would “risk fracturing one of Britain’s greatest public services”.

Union leaders including Billy Hayes, Brendan Barber and Bob Crow, respectively general secretaries of the Communication and Workers Union (CWU), the Trades Union Congress and the National Union of Rail, Maritime and Transport Workers planned to attend a rally in Westminster beginning at 12:00 GMT.

Tony Benn, Ms Smith and fellow Labour backbenchers John McDonnell, Katy Clarke and Michael Connarty were all expected to attend.

Observers say the government faces a real possibility of defeat if it refuses to bow to opposition within the parliamentary Labour party.

But with the Conservatives and Liberal Democrat parties saying they support the move to sell off up to a third of Royal Mail to a foreign firm, this is not guaranteed.

The government announced its support for the move after the Hooper review supported attracting new investment through a sale.

The prime minister’s spokesman said the bill aimed to “secure the future of the Royal Mail”.

“That’s what the report was all about,” he added.

“You cannot cherry-pick the various items from Hooper – you have to take this package as a whole. We need to look at this as a package.”

Since then the public have indicated their opposition to part-privatisation. A poll of 1,000 people carried out for the CWU found nine out of ten against the government selling a percentage of Royal Mail to a foreign company.

Such opposition is likely to be reinforced by worse-than-expected news about Royal Mail’s pensions deficit.

Yesterday evening BBC News cited a letter sent from one of its trustees stating its pensions deficit was significantly greater than the £5.9 million figure in the Hooper review.